Nickel-metal hydride batteries including a hydrogen storage alloy as a negative electrode active material are excellent in output characteristics and high in durability (life characteristics and storage characteristics). For this reason, such nickel-metal hydride batteries have been attracting attention as a power source for electric automobiles and the like. In recent years, however, lithium ion secondary batteries have also been used for this application. Therefore, in view of making the nickel-metal hydride batteries more advantageous, it is desired to improve the output characteristics and the durability of nickel-metal hydride batteries.
For the hydrogen storage alloy, an alloy having a CaCu5 type (AB5 type) crystal structure is mainly used. In view of enhancing the durability, among the alloy having an AB5 type crystal structure, MmNi5, where Mm is a mixture of rare earth elements, has been used, and a part of Ni contained in this alloy has been replaced with another element, such as Co, Mn, Al or Cu.
In view of enhancing the durability as well as achieving a further improvement in capacity, one proposal suggests replacing part of rare earth elements including Y that are located at the A sites with a 2A group element such as Mg, and further controlling the magnetic susceptibility of the hydrogen storage alloy while the battery is in use (see Patent Document 1). Patent Document 1 discloses that, by configuring as described above, a high capacity nickel-metal hydride rechargeable battery having an improved cycle life and an improved discharge capacity at low temperature of about −20° C. can be provided.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-083593